The present invention relates to a method for producing molded bodies containing highly radioactive wastes wherein the wastes are mixed with molten glass or are melted together with glass formers, the resulting melt is converted to glass granules or glass powder and these granules or the powder are embedded in a matrix of pure metal or metal alloys.
The necessity of providing long-term storage for solidified products containing highly radioactive wastes in, for example, salt stocks, brings about the following requirements for these final storage products:
First, the product must be at an internal thermochemical equilibrium, i.e. it must be in a minimum energy state since this is presently the best assurance for thermochemical stability.
Second, the product must be of such consistency that interactions with the environment cannot become a safety risk. Such interactions cannot be completely excluded since, due to the actual conditions of state and the possible changes in these conditions of state over a long period of time, it cannot be assured that an equilibrium remains in effect at the storage location between the final storage product and its environment.
If these requirements are not met, changes in the product may adversely affect the interactions between various components or phase conversions or its properties, such as, for example, heat conductivity, corrosion resistance or strength, and chemical and/or mechanical interactions with the environment, such as leaching or mechanical stresses as a result of geologic pressure and shear forces, may destroy the final storage products wholly or in part. Such a destruction would involve the uncontrollable transfer of highly radioactive fission products into the biosphere.
In order to solidify radioactive wastes for storage, it is well known to treat waste containing aqueous solutions by first reducing the volume of such wastes, thereby concentrating the radioactive substances, and then treating the concentrates by subjecting them together with glass formers to a heat treatment until the radioactive substances become distributed throughout the resulting glass melt, which is solidified into a solid body. Alternatively, the waste containing solution may be denitrated, spray dried, and calcined and the resulting calcinate may then be mixed in solid form with a glass former or with a ground, previously produced glass frit.
In the course of prolonged storage, decomposition of the glass structure produced by the prior art methods may occur due to the continued emission of radiation and heat energy by the incorporated highly radioactive substances. As a result, the resistance of the glass structure to leaking deteriorates with time, and its ability to effectively retain radioactive materials is diminished, as compared with the nondecomposed glass structures which are highly resistant to leaching.
In order to effectively increase the resistance to leaking of the well known waste and glass solidification products of the prior art, and to insure their physical stability for extended periods, German Offenlegungsschrift No. 2,524,169 discloses a process in which a glass melt containing the highly radioactive fission products is initially converted to glass granules and these granules are then filled into metal containers. Then, the empty space between the granules is filled with molten metal or a molten alloy, preferably of lead or lead alloys. This process is not supposed to result in an increase of the bulk volume of the waste granules within the containers.
The surrounding or encasing of the glass granules with metal melts has the grave drawback, however, that a product is obtained in which the glass granules contact one another. Thus, it cannot be excluded that in the process according to German Offenlegungsschfrift No. 2,524,169:
(a) the points of contact of the glass granules with one another react to mechanical stresses which causes constant brittle fracture; and
(b) with respect to corrosion or leaching, there always exists access from the environment to all fission product-containing granules in the interior of the product.
Moreover, in the process according to German Offenlegungsschrift No. 2,524,169, the selection of usable metal melts is limited to those whose wetting with the types of glass employed is satisfactory and whose coefficients of thermal expansion are sufficiently low compared to that of glass that contact at the metal-glass interface remains in existence at all times, even after cooling of the mixture from temperatures above the melting point of the metal.
Contact between glass and metal must be maintained at least to an extent which assures the heat transfer to the metal phase during final storage.